Yosemite’s yellow-legged frogs bounce back from near extinction

Many amphibians worldwide are facing extinction, but one frog in California’s Yosemite National Park is defying the trend. Researchers in the park were surprised to witness a large-scale recovery of the endangered Sierra Nevada yellow-legged frog (Rana sierrae).

The 20-year study, published recently in the Proceedings of the National Academy of Sciences, captured a sevenfold increase in the frogs’ numbers throughout the park. The population made leaps toward recovery after predatory fish and fungal infections declined.

“I wasn’t prepared for the patterns we saw in some lakes to be occurring through the park,” said Roland Knapp, lead author and biologist at the University of California Sierra Nevada Aquatic Research Laboratory.

The frog was once the most common amphibian in the Sierra Nevada, but predatory nonnative trout and infectious chytrid fungus nearly wiped the frog out. The Sierra Nevada yellow-legged frog is now federally endangered in the US, and it is on the IUCN Red List of Threatened Species.

Frogs started coming back when fish numbers went down, according to the study. Since the late 1880s, humans added trout to Yosemite’s previously fish-free lakes. This practice slowed in the 1970s and stopped in 1991. With fewer hungry fish around, more frogs could survive.

Additionally, the previously devastating Chytridiomycosis disease, caused by the chytrid fungus, doesn’t seem to affect yellow-legged frogs as much recently. To test this, researchers took frogs into the lab and infected them with chytrid. Frogs with previous chytrid exposure were better at fighting the disease than frogs seeing the fungus for the first time.

“These frogs are really resilient if you give them half a chance,” said Knapp. “In Yosemite, you can give them high-quality habitat and time. With those two together, these frogs have a remarkable capacity to recover.”

Despite this resilience, the frogs are still limited to about 7 percent of their historic range. There are fewer fish, but they still threaten frogs. The frogs have trouble swimming to new lakes before being gobbled up by trout.

“I’ll feel better when formerly occupied lakes are being recolonized,” said amphibian expert David Wake, professor of biology at the University of California, Berkeley, who was not part of the study. “The frogs are still pushed up against the very edge of their range.”

But Knapp is actively working to help the frogs return. He and other researchers are moving frogs from healthy populations into unoccupied lakes. And continuing to remove fish also gives frogs a fighting chance to expand their range. The scientists feel the frogs can keep recovering with a little help from their friends.

But it’s not clear what the results mean for other amphibians suffering from fungal disease. Knapp and his team don’t know why the frogs can now withstand chytrid. Rapid evolution might play the key role, Knapp believes.

“When all the susceptible frogs die, in one generation you go from more susceptible to less susceptible frogs,” said Knapp. “There’s undoubtedly [natural] selection going on all the time.”

But the frogs could become resistant in other ways. Bacteria on their skin might have grown better at fighting fungus, Knapp said. “We are dying to know what the actual mechanism is and whether it differs between species and even populations,” he noted.

Overall, this study is a rare delight in ecology: the frogs are recovering from near extinction. Perhaps other amphibians have a shot, too.

Citation:
Knapp, R. A., Fellers, G. M., Kleeman, P. M., Miller, D. A., Vredenburg, V. T., Rosenblum, E. B., & Briggs, C. J. (2016). Large-scale recovery of an endangered amphibian despite ongoing exposure to multiple stressors. Proceedings of the National Academy of Sciences, 201600983. doi: 10.1073/pnas.1600983113

Ula Chrobak is a graduate student in the Science Communication Program at the University of California, Santa Cruz. Other Mongabay stories produced by UCSC students can be found here.